777 AIRCRAFT MAINTENANCE MANUAL
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HORIZONTAL STABILIZER CONTROL -GENERAL DESCRIPTION
General
The PFCS commands automatic movement of the stabilizer. The pilots manually command the stabilizer to move.
Flight Deck Controls
The pilots operate two pitch trim switches for manual pitch trim control. The switch modules are on the outboard side of each control wheel. They send electrical pitch trim signals to the ACEs.
Two alternate pitch trim levers are on the captain’s side of the control stand. These levers mechanically control stabilizer movement.
Two guarded cutout switches on the aisle stand control hydraulic shutoff valves on the stabilizer trim control modules (STCMs). When the pilot moves the switches to the cutout position, hydraulic pressure is shut off to the hydraulic motors.
Two stabilizer position indicators on the control stand show the position of the stabilizer. A green band on the indicator shows the correct stabilizer position for takeoff. See the horizontal stabilizer position indication section for more information (SECTION 27-48).
EFFECTIVITY
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Stabilizer Controls
Two stabilizer trim control modules (STCMs) receive commands from the ACEs and the alternate pitch trim levers to control the hydraulic pressure to the motors and brakes.
Two hydraulic motors that use different hydraulic system power sources, cause the ballscrew actuator to rotate. Two hydraulic brakes work with the motors and are powered by the same hydraulic systems. The brakes release and the motor runs with hydraulic pressure. With low pressure, the brakes set and the motors do not run. This prevents movement of the stabilizer.
The horizontal stabilizer is a one piece, negative lift airfoil. It pivots at its rear spar. A ballscrew actuator, attached to the front spar, moves the stabilizer leading edge to a maximum of 4 degrees up and 11 degrees down.
Cables transmit the stabilizer movement to three stabilizer position modules (SPMs) in the stabilizer compartment. The SPMs supply stabilizer position data to the ACEs.
Column Cutout
When the pilot moves the control column in the opposite direction of the pitch commanded by the pitch trim switches, the PFCs cut out the pitch command to the STCMs. This stops the stabilizer movement.
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HORIZONTAL STABILIZER CONTROL -GENERAL DESCRIPTION
Stabilizer Automatic Cutout
In case of uncommanded pitch trim, the PFCs command the hydraulic shutoff valves on the STCMs to close.
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HORIZONTAL STABILIZER CONTROL -GENERAL DESCRIPTION
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HORIZONTAL STABILIZER CONTROL -COMPONENT LOCATIONS -FLIGHT DECK AND BELOW
General
Stabilizer system components are in the flight deck and below the flight deck.
Flight Deck Components
Two pitch trim switches are on the outboard side of each control wheel. The switches have three positions: nose-up, nose-down, and off.
Two alternate pitch trim levers are on the P10 control stand. They connect by cables to mechanical valves on the STCMs.
Two guarded stabilizer cutout switches are on the P10 control stand. Each switch removes hydraulic power from a hydraulic motor and brake when put in the CUTOUT position.
Two stabilizer position indicators are on the P10 control stand. They both have a green band to show when the stabilizer is in the proper position for take-off.
Components Below the Flight Deck
The auto shutdown relays are between flight deck floor beams in the forward equipment center.
The pitch trim control forward quadrants are below the control stand. Access is through the forward equipment center.
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HORIZONTAL STABILIZER CONTROL -COMPONENT LOCATIONS -FLIGHT DECK AND BELOW
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HORIZONTAL STABLIZER CONTROL -COMPONENT LOCATIONS -TAIL
Tail Components
The stabilizer is a single unit. It attaches to the aft fuselage by hinges on the stabilizer rear spar. The forward stabilizer structure attaches to a ball nut on the stabilizer ballscrew. The stabilizer ballscrew is on the airplane structure in the stabilizer compartment. Access to this area is by a service access door on the left side of the lower aft fuselage.
Also in the stabilizer compartment are the three stabilizer position modules (SPMs) assemblies and the two stabilizer trim control modules (STCMs).
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